Rocks with Quartz Veins in Brazil. 345 



A very pure specimen of the granular kaolin kindly analyzed 

 by Dr. Hussak gave : Si0 2 44*96 per cent, A1 2 3 42-09 per 

 cent, H 2 13*25 per cent. This composition differs from that 

 of a typical kaolin derived from a feldspar only in a slight 

 excess of alumina which may perhaps be due to the precipita- 

 tion with it of rare elements that are known to occur in the 

 clay. 



The quartz and kaolin are so blended that they must be 

 considered as constituting a single geological body, and this 

 can hardly be else than a rock of original granitic, or rather 

 pegmatitic type. Moreover this body from the character of 

 its contact, as shown in the figure, must have been eruptive. 

 The character of , the accessory elements is in accord with this 

 view. Those that appear macroscopically are rare prisms of 

 red rntile and handsome tabular crystals and groups of specu- 

 lar iron. The heavy residue obtained by washing is in parts 

 very small, in other parts (where rutile abounds) very abundant, 

 and with the appearance of being wholly authigenetic. It con- 

 sists principally of rutile partly in rather large sagenitic groups 

 of octahedral form that are evidently pseudomorphs after some 

 mineral of the regular system (possibly titaniferous magnetite) 

 with extremely rare grains of anatase, magnetite, tourmaline and 

 xenotime. The latter is in beautiful glassy prisms with short 

 pyramidal terminations of rather large size that show a ten- 

 dency towards crystals of macroscopic dimensions such as occur 

 rarely in the concentrates of the mine. The rutile and anatase 

 are evidently secondary, and of the presumably primary accesso- 

 ries, magnetite, tourmaline and xenotime, the latter is the most 

 significant as thus far it is only known in situ, at least in 

 Brazil, as an extremely constant and characteristic accessory of 

 the ultra-acid (muscovite) granites and pegmatites. The ab- 

 sence (at least apparent, as in order to avoid admixtures only 

 small quantities could be washed) of its almost constant com- 

 panions, zircon and monazite, is noteworthy. In this connec- 

 tion it may be recalled that in a recent paper by Dr. Hussak,*" 

 on an auriferous quartz vein in the same series near Ouro 

 Preto, these three characteristic granitic accessories were found 

 together, and from this circumstance and from the occurrence 

 of characteristic contact minerals the same conclusion was 

 arrived at, viz. : that the vein is an ultra-acid phase of a 

 granitic apophyse. 



The colored clays of the Guia present the appearance of the 

 decomposition products of a micaceous schist. As already 

 stated, the distinct banding of white and colored portions, such 

 as is represented in the figure, in the parts next to the quartz- 

 kaolin vein, is probably due to minute interlaminated projec- 



* Zeifcschr. f. prakt. Geol., Oct., 1898. 



